Despite the ongoing demand for reducing power consumption of circuits, many applications still require high voltage to operate. Such applications include, for example, microelectromechanical systems (MEMS) interfaces, Electrically Erasable Programmable Read-Only Memory (EEPROM) programmers, liquid crystal display (LCD) drivers, and so on. Capacitive charge pumps are used to generate the required high voltage in low power applications, while step-up DC-to-DC converters are used in high power applications.
One issue with conventional charge pump implementations is that such designs suffer from the continuous increase in the threshold voltage from stage to stage due to the increase in the bulk-source voltage. To further illustrate, reference is made to a conventional charge pump 100 shown in FIG. 1. As shown, each switching stage 102a, 102b, 102c has three inputs: a clock, the clock inverse, and the input voltage, and each stage produces an output that is greater than the input voltage of that stage by Vin−δ, where δ is the threshold voltage of the transistors used in the stage if no technique is used to eliminate the threshold voltage loss, and δ equals zero if some technique is used to eliminate the threshold voltage loss. This voltage increase is produced by the pumping capacitors Cpi (i=1, 2, . . . , N) and the switching stages 102a, 102b, 102c. The voltage across the pumping capacitors CP1 in the first stage 102a is Vin−δ. In the second stage 102b, the voltage across the pumping capacitors CP2 increases to 2Vin−2δ.
The voltage across the pumping capacitors continues to increase with each stage such that the voltage reaches NVin−Nδ at the (final) Nth stage. Thus, high voltage capacitors must be used to implement the pumping capacitors in such designs as the one illustrated in FIG. 1. However, high voltage capacitors usually have high dielectric thickness in order to maintain acceptable electric field in the dielectric. This leads to a relatively low capacitance density (capacitance per unit area). Hence, a large area is required to implement the required capacitance for the charge pump. Since the charge pump area is usually dominated by the pumping capacitors, the overall area of the charge pump becomes unacceptably large.